In order to meet the growing demand for internet bandwidth with traffic growth rates around 40-50% per year, telecommunication component providers face the task of increasing the spectral efficiency of fiber utilization. After 10 Gbit/s systems (G-Giga) became successful in the 1990's, solutions for 40 Gbit/s became available in the last years. Standardization and research are now focused on the development of 100 Gbit/s systems with coherent polarization multiplexed (CP) QPSK (Quadrature Phase Shift Keying) being the most likely modulation format for next generation systems. Since polarization multiplexing utilizes both light polarizations, it is possible to send the signal at a rate of ˜25-28 G symbols per second, thus fitting nicely into the standard 50 GHz grid for DWDM (Dense Wavelength Diversity Multiplex) optical systems.
E.g. Seb J. Savory, “Digital filters for coherent optical receivers”, Optics Express 16, No. 2, pp. 804-817, 9. Jan. 2008 describes the principle of a polarisation diversity multiplex (polmux) transmission system with dispersion compensation.